It is somewhat of a surprise to hear that both of Elimelech’s sons married foreign women. It would have been expected that arrangements be made to have at least one of them marry a girl from “back home,” especially in the case where the family still clearly regarded Bethlehem in Judah as the place where they ultimately belonged (1:19,22). They had not even resided in the land of Moab all that long, i.e., ten years at the maximum. It is no doubt true to say that cross-tribal marriages were not the norm for the people of Judah either, at least not among those who took their religion seriously, but then again, the receptor would probably not know that information. There are many reasons why intertribal marriages are generally frowned upon by the Tonga, for example. Social and religious factors are especially prominent, e.g., differences in liminal customs at birth, maturation, marriage, and death; diverse religious practices, such as how to sacrifice and to whom (which ancestors); and so forth.

Source: Wendland 1987, p. 167.